FOnline 2238, both server and client doesn't require .NET Framework.
some tools ( like the FO2238Config.exe ) requires .NET Framework, but you don't need these tools to run the server or client.
i don't know what problem you have with the client, but i run it both server and client on a Windows XP OS without problems, so the OS isn't the problem ( and, BTW, i don't have .NET Framework installed ).